The dissertation is presented in cumulative form and consists of four individual manuscripts, which are referred to by their corresponding Roman numerals in the text. All manuscripts are completely reproduced in Appendix A-D. Two manuscripts are published. The others are submitted and still in the review process.

iii Acknowledgements At this point I would like to thank the following people who have contributed significantly to the success of this work.

I would like to express my deep and sincere gratitude to Prof. Dr. Dieter Scherer. His understanding, encouraging and personal guidance have provided the best basis for the present thesis. He let me participate in a number of fruitful discussions, excursions, expeditions and scientific conferences. Without his support I would not be so enthusiastic about the scientific issues of urban climatology and thermal remote sensing.

I wish to express my warm and sincere thanks to Dr. Jochen Richters, Prof. Dr. Andreas Christen and Prof. Dr. Eberhard Parlow for their co-supervision, motivation and support in pursuing my PhD thesis.

Many thanks to all colleagues at the chair of climatology for creating such a friendly place where it is a pleasure to work. I cordially thank Marco Otto for reviewing all my manuscripts, his support with numerous and helpful comments, as a discussion partner and friend. Special thanks go to Hartmut Küster who helped substantially to set up and maintain the meteorological and thermography experimental sites. I am grateful to Roman and Fabi for discussing various scientific issues and for finding quick and reliable solutions for many practical problems.

I am immensely grateful to my parents and my wife Sabi. Her lovely support always encouraged me. I am also grateful to all my friends, especially Olli for reviewing my manuscripts and all for being there and helping me to keep perspective.

viii 1. Introduction Although cities themselves form a very small fraction of the global surface area, the urban environment affects a considerable portion of the global population. Today fifty percent of the population lives in cities or urban agglomerations (UN 2010). Urbanisation is set to continue i.e. by 2050 the global percentage of urban dwellers is projected to reach 70.1 % and in the case of Europe to 84.3 % (UN 2010). Understanding the urban climate and its anthropogenic modifications is therefore of great interest in the creation of a healthy and comfortable environment to which an increasing amount of urban dwellers are exposed. Urbanisation has led to distinct landscape changes, which typically involved the substantial replacement of natural cover by materials, which are generally impermeable and have distinct thermal and radiative properties. Moreover, this new landscape has a unique geometry associated with building form and arrangement that generates atmospheric turbulence and interferes with radiative exchange processes. These landscape changes and anthropogenic emissions affect the exchange of heat, mass and momentum between the surface and the atmosphere resulting in the development of an urban climate that deviates from the surrounding non-urbanised areas (Landsberg 1981, Oke 1988). One of the most distinct alterations is the characteristic warmth of urban areas compared to their surroundings referred to as the urban heat island (UHI), which is one of the best-documented anthropogenic climate modifications (Arnfield 2003). The concept of scale is important to understand the phenomena of urban climate (Oke 1984). The thesis answers the question in which way individual elements of the urban surface interact with the adjacent atmosphere. This leads to a microscale perspective. Every surface and object has its own microclimate. Typical scales extend from < 1 m to hundreds of metres (Oke 2006). ‘Surface and air temperature may vary by several degrees in very short distances, even millimetres, and airflow can be greatly perturbed by even small objects’ (Oke 2006, p. 3). The urban surface is a patchwork of vertical and horizontal elements, such as buildings that consist of walls and roof facets, each with a differing time-varying exposure to short- and long-wave radiation and ventilation (Arnfield 1990; Kobayashi and Takamura 1994; Blocken and Carmeliet 2004). Moreover, there are sealed surfaces (Asaeda et al., 1996; Anandakumar 1999), irrigated gardens and lawns (Oke 1979; Suckling 1980; Spronken-Smith et al. 2000) and trees (Oke 1989; Grimmond et al. 1996; Kjelgren and Montague 1998). Single surface facets (microscale γ) may be aggregated hierarchically to define the urban canyon (microscale β). Urban canyons and roofs of adjacent buildings define city blocks (microscale α), which in turn scale up to neighbourhoods (localscale), land-use zones (mesoscale γ) - 1 -